In the exhaust duct of a conventional aerospace gas turbine engine a plurality of vanes or struts are attached to the duct casing and extend between walls of the duct, to support the duct and to maintain its shape. These so-called “exit guide vanes” are disposed in the path of hot exhaust gases from the engine and so are prone to becoming very hot themselves. To combat the effects of overheating, the vane is designed as a hollow structure which allows the flow of cooling air in its interior.
It is known to provide cooling air flows to stators and rotor vanes using high pressure air bleeds drawn from the engine's compressor. Due to the relative pressure drop between the compressor bleed and the region to which the cooling air flow is provided, highly effective heat transfer can be achieved, albeit with high pressure losses.
Where a significant pressure drop does not exist, highly effective heat transfer features with high pressure losses cannot be used. Turning the flow and adequately conditioning it to maximise cooling effectiveness is therefore difficult. Where the hot core flow impinges onto the vane leading edge tip, its high static pressure further decreases the available pressure difference relative to the bypass air. Film cooling the hot surface of the vane leading edge is therefore impossible.
Whereas the high pressure drop between the compressor supply and the coolant exit makes for very effective cooling in vanes which are actively cooled in this way, vanes which are cooled with scooped bypass air, at lower pressure, are less effectively cooled. The reason that so-called scoop-fed vanes are less effectively cooled is that the flow of cooling air is relatively low and little or no attempt has been made to control the internal flow path. The pressure drop available is insufficient to ensure that the cooling air will change direction sufficiently to flow along the internal surfaces of the vane. Accordingly, there are areas of the vane that are not cooled, or are cooled insufficiently. This can lead to uneven thermal expansion of certain parts and possibly overheating of the internal load bearing structure, by heat convection and radiation.
An example of the cooling arrangement of a vane in the turbine section of an engine, utilising only low pressure bypass air is provided in GB2467790B.